Finally some good news for Chipotle

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Chipotle is finally getting some good news: The Centers for Disease Control and Prevention has declared the end of the dual E. coli outbreaks tied to its restaurants.

The outbreaks sickened 55 people in 11 states and sent the company's sales plunging since October.

"These two outbreaks appear to be over," the CDC said Monday.

Chipotle reports fourth-quarter earnings after market close on Tuesday. The company has said same-store sales, or sales at stores open at least a year, declined by 30% in December.

Even though the outbreaks appear to be over, the sales declines will most likely continue for at least another year.

Credit Suisse analysts looked at food-safety scares at McDonald's China, Taco Bell, Jack in the Box, and Yum Brands and how they affected sales at those brands.

In each case, analysts wrote, same-store sales remained depressed for four quarters then turned positive when the event quarter, or the time period in which the food-safety scare was first reported, was lapped.

This chart shows the same-store-sales trends following the food-safety scares that were studied:

Credit Suisse
The analysis suggests that Chipotle's same-store sales will remain negative through the third quarter of 2016, "barring a heroic promotional/marketing effort to win back customers in the coming months," the analysts wrote.

Credit Suisse is predicting an overall same-store-sales decline of 7.4% for 2016.

In most cases the brands made a full recovery in the years after the food-safety scares. But there's a key issue in Chipotle's outbreak that didn't exist for any of the other companies.

"One key concern/difference in the [Chipotle] situation is that the source of the E coli outbreak(s) has yet to be found," the analysts wrote. "This likely makes the recovery process more difficult for [Chipotle]. In all of the five historical cases just mentioned, the cause of the food safety issue was determined relatively quickly."

The analysts studied a 1993 E. coli outbreak at Jack in the Box related to undercooked beef, a 2006 E. coli outbreak at Taco Bell related to shredded lettuce, improper use of antibiotics by two chicken suppliers for Yum China in 2012, and unsafe food-handling practices at a Chinese supplier that affected Yum China and McDonald's China in 2014.